WordPress Custom Taxonomy – If not parent term

You can use get_term_children for checking there is have any child term or not.

$taxonomy = 'custom_tax';
$term = get_queried_object();
if ( count( get_term_children( $term->term_id, $taxonomy ) ) === 0 ) {
     echo '<p>HTML only terms without child terms</p>';
}
else
{
    echo '<p>HTML only terms with child terms</p>';
}